152 DESCRIPTIVE GEOLOGY. 



color, carrying argillaceous arid ferruginous material, with impressions of 

 deciduous leaves and stems. Above these occur massive white sandstones. 



Valley of the North Platte. — Along the bluffs between Elk Mount- 

 ain and Saint Mary's Peak occurs another anticlinal fold. To the northwest 

 of Elk Mountain, along Pass Creek, the beds fall away gradually to the 

 west and southwest, bending in strike as they go northward, until south of 

 Wolcott Station, their dirction is a few degrees north of west and south of 

 east, the course of Pass Creek roughly indicating the outlines of the uplift. 

 Just north of Pass Creek occurs a very sharp, clearly-defined anticlinal fold, 

 the beds on the upper side dipping north and northeast. 



North of the railroad, and on the east side of the North Platte, the 

 Fox Hill sandstones form a prominent monoclinal ridge, dipping northeast, 

 which below Fort Steele determined the course of the river in a similar 

 manner as beds of the same horizon have marked out the course of the 

 Medicine Bow River. At the southern end of this ridge, at Saint Mary's 

 Peak, the beds occur considerably disturbed, and metamorphosed into a 

 hard sandstone, with a strike north 15° east, and a dip 16° east. 



The ridge on the south side of the fold mentioned as north of Pass 

 Creek crosses the North Platte River, in a nearly due east and west line, 

 about 2 miles above Fort Steele. It is formed of sandstone beds, which dip 

 45° to 50° to the southward. This ridge presents singular narrow crests of 

 the harder upright strata, forming straight ridges only a few feet in width, 

 and enclosing narrow monoclinal valleys worn out of the more yielding 

 beds. It extends to the westward in an east and west line for nearly 10 

 miles, forming the southern boundary of the open Quaternary valley west 

 of Fort Steele, then gradually trends to the southward with a constantly- 

 decreasing dip to the south and east, joining the Sage Creek Bluffs. The 

 latter form the line of bluffs which border Sage Creek on the north, 

 possessing an easterly strike and a dip northward, which gradually shal- 

 lows passing eastward, until near the mouth of Sage Creek they are 

 practically horizontal. 



On the surface of the flat country to the north of this line of bluffs, 

 where the soil accumulation is very slight, the jointing planes of these 

 sandstones present a line of cracks of remarkable regularity; the grass 



